In the past, a bicycle frame joint was constructed of iron pipe joined by welding or brazing. More recently, for lightening the weight of a bicycle frame, the pipes have been made by fiber reinforced plastics.
However, the pipes made by fiber reinforced plastics cannot be joined by welding or brazing, so thermosetting adhesive agent has been spread around the cylindrical projecting part of a lug, and the pipe heat fitted outside or inside of said cylindrical projecting part as shown in FIG. 6 and FIG. 7, at a temperature of from 120.degree. to 150.degree. C.
A clearance of 0.05 to 0.15 mm must be maintained between the projecting part of the lug and pipe so that the joint can be made.
Since the thermal expansions of the projecting part of the lug made by light alloy and the pipe made by fiber reinforced plastics are different, unbalanced stress remains in the joint when the joint returns to ambient temperature.
Consequently, the joint is distorted by stress from every direction, and as a result thereof, the adhesive agent spread between said pipe and projecting part of said lug is torn apart.
The user, thus cannot rely on the strength of the bicycle frame.
In Japanese Utility Model application laid open gazette No. 1979-176,955 and Japanese Patent laid open gazette based on PCT application No. 1983-501,904 it is suggested to cover the joint of the pipes of a bicycle frame with plastics.
The joint does not comprise a projecting part of a lug, and is covered and fixed only by plastics, so the strength of the frame is insufficient unless the plastics layer covering the joint is very thick.
If the plastic layer is thin, the tensile stress and elasticity of the plastics cannot mounting a tight joint during operation of a bicycle.
On the other hand, if the plastics layer would be very thick, the bicycle frame is not light-weight.